[llvm-commits] [llvm] r169459 - in /llvm/trunk: include/llvm/Target/TargetLowering.h lib/CodeGen/SelectionDAG/SelectionDAG.cpp lib/CodeGen/SelectionDAG/TargetLowering.cpp lib/Target/ARM/ARMISelLowering.cpp lib/Target/ARM/ARMISelLowering.h lib/Target/X86/X86ISelLowering.cpp lib/Target/X86/X86ISelLowering.h test/CodeGen/ARM/extload-knownzero.ll

[Duncan Sands]

Hi Evan,

On 06/12/12 02:28, Evan Cheng wrote:
> Author: evancheng
> Date: Wed Dec  5 19:28:01 2012
> New Revision: 169459
>
> URL: http://llvm.org/viewvc/llvm-project?rev=169459&view=rev
> Log:
> Let targets provide hooks that compute known zero and ones for any_extend
> and extload's. If they are implemented as zero-extend, or implicitly
> zero-extend, then this can enable more demanded bits optimizations. e.g.

is this correct?  All kinds of DAG combines exploit that anyext means "any",
eg: any_extend(trunc X) -> X.  On a target where any_extend does zero-extend,
won't your patch say that all the upper bits of "any_extend(trunc X)" are
zero, causing all kinds of oddness when they turned later into the bits of X,
which may not be zero.

Ciao, Duncan.